This invention relates to solid-state lasers, and more particularly to solid-state laser arrangements capable of fast wavelength or frequency tuning (scanning).
Microwave and millimeter-wave electrical signals are desirable for various types of range detection, medical and communications equipment. Ranging equipment or radar for automobile collision prevention and for medical applications such as small tumor detection has been under development, and such equipment often uses millimeter-wave radiation, which is more suitable for short-range ranging than are longer wavelengths, at least in part because the equipments required to process millimeter-wave signals tend to be quite small and light in weight, and also because the millimeter waves provide higher accuracy. One type of radar system which has been used for automotive collision detection is swept-frequency radar, in which the frequency of the transmitted radiation recurrently varies in a linear manner between a low frequency and a high frequency. It should be noted that those skilled in the art know that frequency and wavelength are related by the speed of light, so reference to frequency or frequencies is also a reference to wavelength or wavelengths. This frequency scanning is known as xe2x80x9cchirpxe2x80x9d or xe2x80x9cchirping,xe2x80x9d a reference to the sound made by an audio tone performing the same kind of frequency scanning.
In a ranging radar system, a portion of the transmitted signal is reflected by the target, and returned to the radar transmitter. At the transmitter, the timing of the returned signal is compared with the time of the transmitted signal in order to determine the range or distance at which the target was when the signal was reflected. In a pulse radar system, the transmitted signal is in the form of recurrent pulses of energy, and the time lapse between the transmitted pulse and the received reflected signal is a measure of the distance. In swept-frequency or chirp systems, signals are transmitted continuously or almost-continuously, at least during selected intervals. The time lapse between the transmitted signal and the received reflected signal is determined by comparing the current frequency of the transmitter signal with the frequency of the received reflected signal. The frequency difference between the current transmitter frequency and the frequency of the returned signal is a measure of the time difference, and therefore of the range. The signal processing for such a system is simplified if the each of the recurrent frequency sweeps is linear with time, so that a given frequency difference always represents the same target range; a small frequency difference corresponds to a short target range, and a large frequency difference corresponds to a large target range.
In the context of radar for automotive collision monitoring or control, the ranges are relatively short, with the maximum range being on the order of a hundred feet or less, corresponding to round-trip signal transit durations (also known as radar range) of about 0.2 microsecond, or 200 nanoseconds (ns). Thus, the maximum round-trip transit time is about 200 nanoseconds, and the system must respond to changes in range at much shorter distance, which is to say that the range resolution must be good. Good range resolution, in turn, implies a large frequency excursion of the swept signal. Also, high signal-to-noise ratio is desirable. One of the problems attendant on the use of swept-frequency radar systems is that the frequency difference between the transmitted signal and the received signal tends to be relatively small at short target ranges. This frequency difference is the signal with which signal processing begins in order to determine the range. The frequency difference for short-range targets arises because of the difficulty of achieving a large frequency scan or sweep within the short time before the return signal arrives.
It is an inconvenient fact that solid-state amplifiers tend to have inherent noise which follows a 1/f or inverse frequency characteristic. That is, the inherent noise of the devices is greatest at low frequencies, and decreases at high frequencies. Thus, it is desirable to receive a relatively high-frequency range-representative signal at the beginning of signal processing in order that the processing itself not add to the noise already extant in the returned signal. This problem may be addressed by measuring only longer ranges, or by reducing the signal processing noise.
Improved systems are desired.
A laser arrangement according to an aspect of the invention includes a first solid-state laser formed on a chip. For convenience, this laser arrangement is referred to a xe2x80x9cchirpxe2x80x9d or xe2x80x9cchirpedxe2x80x9d laser arrangement, although some embodiments do not actually frequency scan. The first solid-state laser has a particular optical or electrooptical cavity length, and at least a portion of the cavity of the first solid-state laser includes electrooptic material. The chirp laser arrangement also includes a second solid-state laser, also formed on the same chip, and having the same or particular optical or electrooptical cavity length. At least a portion of the cavity of the second solid-state laser also includes the electrooptic material. Ideally, the first and second solid-state lasers are as identical as can be achieved by the use of batch processing, and the cavities are arranged, as known in the art, so that only one (or possibly a few) longitudinal modes are generated. The purpose of forming the lasers on the same chip is to allow them to be made as identical as possible, which also has the salient advantage of coupling them together thermally, so that changes in the environmental temperature tends to affect both the first and second lasers equally. The chirped laser arrangement includes first and second optical pumps having nominally the same pump frequency, and an optical coupling arrangement or means coupled to the first and second optical pumps and to the first and second solid-state lasers, for pumping the first and second lasers with similar pump light, so that, or whereby, the first and second lasers produce first and second laser light beams. If it were possible to make the structures identical, the laser light pump beams should be at the same wavelength or frequency. In one avatar of the invention, the first and second pump light sources are in the form of a single pump light source, with an optical power divider dividing the pump power so as to apply equal powers to the first and second lasers, this tends to apply changes in pump power or wavelength equally to the first and second lasers, so that the frequencies or wavelengths of the first and second light beams tend to track each other notwithstanding changes in the pump source parameters. The chirped laser arrangement also includes a nonlinear light-to-electric converter, such as an photodetector (an electrooptic diode), which, in the presence of plural light signals at different frequencies or wavelengths, generates electrical signals at frequencies related to the difference. More specifically, if two laser light beams impinge on the photodetector diode, an electrical signal is produced which is at a frequency equal to the difference between the frequencies of the two light beams, and if it should happen that the frequencies of the two light beams were identical, the resulting electrical signal at the output of the electrooptic diode would be zero frequency or xe2x80x9cdirect current.xe2x80x9d The chirp or chirped laser arrangement also includes a second optical coupling arrangement or means coupled to the first and second lasers, and coupled to the light-to-electric converter or photodiode, for coupling the first and second laser light beams to the light-to-electric converter or photodetector, whereby the light-to-electric converter generates at least one electrical difference signal. According to one aspect of the invention, in which the ranging system is a swept-frequency radar type, the second coupling path may include, for example, an optical power combiner or directional coupler such as a star coupler, together with optical fibers extending from the light output ports of the first and second lasers to input ports of the optical power combiner for carrying the first and second laser light beams to the optical power combiner, directional coupler or star coupler, and another optical fiber extending from an output port of the optical power combiner, directional coupler or star coupler to the photodiode or electrooptic diode, for carrying the two laser beams to the electrooptic diode; in this radar context, the electrical difference frequency generated at the diode is then amplified, if necessary, and transmitted as an electromagnetic signal. According to another aspect of the invention, the context is a lidar (light detection and ranging) system, in which the second optical coupling path includes the optical combiner or directional coupler (star coupler), and the light paths between the first and second lasers and the optical power combiner, directional coupler or star coupler, but in which the combined first and second laser light beams at the output of the optical combiner are transmitted over the path to be measured, and reflected by the intended target. In this lidar context, the second optical coupling path also includes a receiving arrangement for picking up or sensing the reflected first and second light beams, and for conveying the reflected and returned first and second light beams to the nonlinear light-to-electric converter or photodetector. The photodetector then converts at least the first and second returned light signals into an electrical difference signal, and further processing can be performed by electronic means to determine the range of the target. According to a further aspect of the invention, an electrode is associated with at least a portion of the electrooptic material of the second laser, for, when electrically energized, electrooptically affecting the length of the cavity of the second laser, thereby affecting the wavelength of the laser light beam of the second laser, which in turn affects the frequency of the electrical difference signal at the output of the photodiode or other nonlinear light-to-electric converter.
In particularly advantageous versions of the lidar and radar embodiments, a constant electrical value, as for example a constant voltage, is applied to the electrode of the second laser, to thereby produce a constant frequency or wavelength offset of the second laser light beam relative to the first laser light beam. In a particular manifestation of these versions, the electrical value applied to the electrode coupled to the electrooptic portion of the second laser cavity is selected so that the nominal electrical difference frequency at the output of the light-to-electric converter is closer to those frequencies at which the electronic processing devices have lowest noise. This improves the signal-to-noise ratio, thereby allowing better range resolution than if no constant electrical value were applied to the electrode of the second laser. Of course, the difference frequency may be selected to be some other value based upon some criterion other than signal-to-noise ratio.
In yet a further manifestation of the invention, the first laser also includes an electrode coupled to the electrooptic portion of its cavity. The constant electrical value is applied to one of the first and second electrodes to provide a constant frequency or wavelength offset between the first and second laser light beams, and a ramp-like or information electrical signal(s) is applied to the other of the first and second electrodes. This has the advantage of reducing electrical coupling between the ramp- or information-signal source and the constant-value electrical source, while allowing the first and second laser light beams to be mutually offset and modulated in a useful manner.